Demineralizer systems for removing ionized particles from water for the purpose of purification have been known in the prior art for a considerable period of time. In such systems, untreated water is purified by flowing it through beds consisting of cation and anion exchange resins. Two types of resin beds may be employed, primary beds containing either a cation or an anion exchange resin, but not both, and mixed beds containing both cation and anion exchange resin beads mixed together. Frequently, systems will employ multiple vessels or tanks containing primary resin beds for performing initial demineralization, followed by a final stage of treatment in a vessel containing a mixed resin bed. The treatment capacity of such a train of demineralizer stages is thus principally limited by the estimated exchange capacity of the primary bed tanks, i.e. fluid volume treated per internval between resin regeneration, for a given influent water composition.
Characteristically, exchange capacity is defined on the basis of user needs and then translated into the volume of cation and anion resins required to provide the desired performance. Once the required resin volume has been established, it is then possible to determine the sizes of the vessels required for containing the resin beds. According to conventional practice, ion exchangers consist of a plurality of vessels in which the actual ion exchange process takes place, together with associated control apparatus, piping and valving. Each vessel comprises a vertical or upright tank housing a bed of exchange resin material therein. One or more distributors are provided within the upper region of such a vessel and a collector system is provided in the bottom region thereof. Since the resin expands during backwashing, and as it goes from regenerated to exhausted form, a significant portion of the volume within the vessel is empty of resin during the in-service, ion-exchange procedure, to allow sufficient "rise space" to accommodate such expansion.
According to the conventional system set out above, both the service and regeneration fluid flows are in a downward direction. That is, fluid is introduced into the top of the exchange vessel and flows downward therethrough. The resin in the bed may either expand during regeneration and shrink during exhaustion, or vice versa, depending on the characteristics of the particular resin selected. When multiple distributors are used, the regenerant is introduced through one, with the other being employed for the service and backwash flows.
Another prior art system is characterized by so-called counterflow or "counter-current" operation. According to this principle, the influent liquid to be treated flows downward through the resin bed and regeneration is accomplished by an upward flow, or vice versa.
The flow of the influent liquid to be treated through the resin bed, gives rise to ion exchange zones which are displaced accordingly through the exchange material as the resin bed becomes progressively exhausted. In other words, ions which are most easily trapped by the resin are removed from the fluid in the first portions of the bed. Less easily captured ions which are more loosely bound are displaced from the resin by the more easily captured, tightly bound ions and do not find exchange sites until they reach positions in the latter portions of the bed. When a sufficient number of exchange sites on the resin have been exhausted by trapped ions, efficient purification is no longer possible. Liquid will pass through the bed untreated. At this point, it is necessary to terminate processing and to backwash the resin bed to remove suspended matter. Regeneration and rinsing of the resin are then accomplished by bringing suitable chemical solutions into contact with the resin, to chemically strip the trapped materials from the resin beads, and then rinsing out the excess regenerant and the impurities.
Regardless of which of the foregoing operational schemes is employed, it is necessary to allow sufficient volume in the vessel to accommodate chemical swelling of the resin bed. In prior art systems with resin volumes and vessel size seleced for specific site requirements, such chemical swelling is accommodated in various ways. For example, in the conventional downflow service vessel wherein downflow regeneration is practiced, as allowance is made for chemical swelling by providing sufficient volume within the vessel to permit upward backwashing of the resin, within the vessel, prior to regeneration. Since backwashing requires that the packed resin beads be agitated apart to free trapped materials and expose the surfaces of the beads in preparation for regeneration, the volume expansion of the bed associated with backwashing is generally several times that associated with the aforementioned chemical swelling. In some types of counterflow schemes, involving upflow regeneration in a downflow service vessel, an allowance for backwash expansion will also be sufficient to provide for chemical swelling. However, in both of these cases, unless the service vessel has been deliberately oversized, additional resin cannot be loaded into the vessel without eliminating needed rise space. And, indeed, such oversizing would be economically inefficient, in any event.
In a contrasting type of counterflow system, the resin bed is compressed upwardly against a retaining collector either during service or regeneration, and backwashing in the service vessel is not practical per se. Therefore, a certain amount of vessel volume must be provided as rise space to accommodate resin swelling. Should additional rise space be provided to permit expansion during backwash, the resin-retaining upward flow collector would not permit the passage of dirt from the vessel. Therefore, this type of counterflow application requires the periodic removal of all of the resin from the service vessel, to cleanse it of foreign material. With this design, overall vessel height is restricted to only that amount needed to contain the resin volume, to accommodate swelling, and to ensure hydraulic efficiency in either the upflow service or regeneration steps. To load additional resin in the vessel in excess of the amount needed for the specific requirements at the time of installation would, therefore, require that the service vessel be made larger than needed, which would be economically wasteful and detrimental to hydraulic efficiency.